According to the prior art, carbon nanotubes are understood as being mainly cylindrical carbon tubes having a diameter of from 3 to 100 nm and a length that is a multiple of the diameter. These tubes consist of one or more layers of ordered carbon atoms and have a core that differs in terms of morphology. These carbon nanotubes are also referred to as “carbon fibrils” or “hollow carbon fibers”, for example.
Carbon nanotubes have been known for a long time in the specialist literature. Although Iijima (publication: S. Iijima, Nature 354, 56-58, 1991) is generally considered to have discovered nanotubes, such materials, in particular fibrous graphite materials having a plurality of graphite layers, have been known since the 1970s or early 1980s. The deposition of very fine fibrous carbon from the catalytic decomposition of hydrocarbons was described for the first time by Tates and Baker (GB 1469930A1, 1977 and EP 56004 A2, 1982). However, the carbon filaments produced on the basis of short-chained hydrocarbons are not described in greater detail in respect of their diameter.
The production of carbon nanotubes having diameters of less than 100 nm was described for the first time in EP 205 556 B1 or WO A 86/03455. In this case, the production is carried out using light (i.e. short- and medium-chained aliphatic or mono- or bi-nuclear aromatic) hydrocarbons and an iron-based catalyst, on which carbon carrier compounds are decomposed at a temperature above 800 to 900° C.
The methods known today for the production of carbon nanotubes include arc discharge, laser ablation and catalytic processes. In many of these processes, carbon black, amorphous carbon and fibers having large diameters are formed as by-products. In the case of the catalytic processes, a distinction can be made between deposition on supported catalyst particles and deposition on metal centers formed in situ and having diameters in the nanometer range (so-called flow processes). In the case of production by the catalytic deposition of carbon from hydrocarbons that are gaseous under reaction conditions (CCVD; catalytic carbon vapor deposition hereinbelow), acetylene, methane, ethane, ethylene, butane, butene, butadiene, benzene and further carbon-containing starting materials are mentioned as possible carbon donors.
The catalysts generally contain metals, metal oxides or decomposable or reducible metal components. For example, Fe, Mo, Ni, V, Mn, Sn, Co, Cu and others are mentioned as metals in the prior art. Although most of the individual metals have a tendency to form nanotubes, high yields and low amorphous carbon contents are advantageously achieved according to the prior art with metal catalysts that contain a combination of the above-mentioned metals.
According to the prior art, particularly advantageous systems are based on combinations that contain Fe or Ni. The formation of carbon nanotubes and the properties of the tubes that are formed are dependent in a complex manner on the metal component, or combination of a plurality of metal components, used as catalyst, the support material used and the interaction between the catalyst and the support, the starting material gas and partial pressure, the admixture of hydrogen or further gases, the reaction temperature and the dwell time or the reactor used. An optimization represents a particular challenge for a technical process.
It is to be noted that the metal component used in the CCVD and referred to as the catalyst is consumed in the course of the synthesis process. This consumption is attributable to a deactivation of the metal component, for example owing to the deposition of carbon on the entire particle, which results in complete coverage of the particle (this is known to the person skilled in the art as “encapping”). Reactivation is generally not possible or is not economically expedient. In many cases, a maximum of only a few grams of carbon nanotubes per gram of catalyst are obtained, the catalyst here comprising the totality of the support and the catalyst used. On account of the described consumption of catalyst, a high yield of carbon nanotubes, based on the catalyst used, is a fundamental requirement of the catalyst and the process.
For the industrial production of carbon nanotubes, for example as a constituent for improving the mechanical properties or conductivity of composite materials, it is desirable, as in all industrial processes, to have a high space-time yield while retaining the particular properties of the nanotubes and minimizing the energy and fuel that are to be used. Applications based on the laser ablation of carbon often yield only low production rates and high contents of amorphous carbon or carbon black. In most cases, such systems on the laboratory scale, with production rates of a few grams per day, can be converted to an industrial scale only with difficulty. Laser ablation is also expensive and a scale-up is difficult. Although various processes described in the literature for the production of carbon nanotubes by CCVD show the suitability of various catalysts in principle, they often exhibit only low productivity, however.
Various processes and catalysts are known in the patent literature for the production of carbon nanotubes. EP 0205 556 A 1 (Hyperion Catalysis International) describes such carbon nanotubes which are produced by means of an iron-containing catalyst and the reaction of various hydrocarbons at high temperatures above 800 to 1000° C. Shaikhutdinov et al. (Shamil' K. Shaikhutdinov, L. B. Avdeeva, O. V. Goncharova, D. I. Kochubey, B. N. Novgorodov, L. M. Plyasova, “Coprecipitated Ni—Al and Ni—Cu—Al catalysts for methane decomposition and carbon deposition I.”, Applied Catalysis A: General, 126, 1995, pages 125-139) mention Ni-based systems as being active in the decomposition of methane to carbon nanomaterials.
In CA 2374848 (Centre National de la Recherche Scientifique, FR) there is disclosed as a possible process for the mass production of carbon nanotubes a process in which a yield of 3 g of CNTs/g of catalyst is achieved using acetylene as carbon donor on a cobalt catalyst. This comparatively very low yield makes the process appear uncritical with regard to ensuring thorough mixing but requires expensive purification steps to obtain a product suitable for use.
Mauron et al. (Ph. Mauron, Ch. Emmenegger, P. Sudan, P. Wenger, S. Rentsch, A. Züttel, “Fluidised-bed CVD synthesis of carbon nanotubes on Fe2O3/MgO”, Diamond and Related Materials 12 (2003) 780-785) also achieve only very low yields (max. 0.35 g of CNTs/g of catalyst) in the production of CNTs from isopentane or acetylene on an iron catalyst. For that reason they also do not discuss possible difficulties during thorough mixing in the reactor during the growth process of the agglomerates.
EP 1399384 (Institut National Polytechnique, Toulouse, FR) describes the production of carbon nanotubes in a CCVD process with an upstream reactor for in-line catalyst production, wherein the catalyst can have a mean particle size of from 10 μm to 1000 μm and can reach a volume increase of the agglomerates of up to twenty times the catalyst amount. With regard to fluidization, it is merely required that the superficial gas velocity in the reactor remains above the minimum fluidization velocity of the particle collective in the reactor and below the gas velocity required for the formation of a plug flow.
In a dissertation by Nijkamp (Utrecht University/NL, 2002, “Hydrogen Storage using Physisorption Modified Carbon Nanofibers and Related Materials”), the production of carbon nanotubes by means of nickel-containing catalysts and methane as carbon donor is described. However, only the laboratory scale is considered therein (reactor inside diameter 25 mm) and, with a relatively low yield (27 g of CNTs/g of catalyst) overall, only a very small amount of material (10-30 g) is produced. The material so produced must be purified before it is used further because the catalyst residues have a disruptive effect in most applications and nickel must not pass into the end products owing to its carcinogenic action.
In the various processes using different catalyst systems that have been mentioned hereinbefore, carbon nanotubes having different structures are produced, and they can be removed from the process predominantly in the form of carbon nanotube powder.
Conventional structures of such tubes are those of the cylinder type. In the case of cylindrical structures, a distinction is made between single-wall monocarbon nanotubes and multi-wall cylindrical carbon nanotubes. Conventional processes for their production are, for example, arc discharge, laser ablation, chemical vapor deposition (CVD process) and catalytic chemical vapor deposition (CCVD process).
Such cylindrical carbon nanotubes can also be prepared by an arc discharge process. Iijima, Nature 354, 1991, 56-58 reports on the formation, by the arc discharge process, of carbon tubes consisting of two or more graphene layers which are rolled up to form a seamless closed cylinder and are nested inside one another. Chiral and achiral arrangements of the carbon atoms along the longitudinal axis of the carbon fibers are possible depending on the rolling vector.
WO 86/03455A1 describes the production of carbon filaments which have a cylindrical structure with a constant diameter of from 3.5 to 70 nm, an aspect ratio (ratio of length to diameter) of greater than 100 and a core region. These fibrils consist of a large number of interconnected layers of ordered carbon atoms, which are arranged concentrically around the cylindrical axis of the fibrils. These cylinder-like nanotubes were produced by a CVD process from carbon-containing compounds by means of a metal-containing particle at a temperature of from 850° C. to 1200° C.
A process for the production of a catalyst which is suitable for the production of conventional carbon nanotubes having a cylindrical structure has also become known from WO2007/093337A2. When this catalyst is used in a fixed bed, relatively high yields of cylindrical carbon nanotubes having a diameter in the range from 5 to 30 nm are obtained.
A completely different way of producing cylindrical carbon nanotubes has been described by Oberlin, Endo and Koyam (Carbon 14, 1976, 133). Aromatic hydrocarbons, for example benzene, are thereby reacted on a metal catalyst. The resulting carbon tube exhibits a well-defined, graphitic hollow core which has approximately the diameter of the catalyst particle, on which there is further, less graphitically ordered carbon. The authors suppose that the graphitic core is formed first by rapid catalytic growth, and then further carbon is deposited pyrolitically. The entire tube can be graphitized by treatment at high temperature (2500° C.-3000° C.).
Most of the above-mentioned processes (arc discharge, spray pyrolysis or CVD) are used today for the production of carbon nanotubes. The production of single-wall cylindrical carbon nanotubes is very expensive in terms of apparatus, however, and proceeds according to the known processes with a very low formation rate and often also with many secondary reactions, which result in a high proportion of undesirable impurities, that is to say the yield of such processes is comparatively low. For this reason, the production of such carbon nanotubes is still extremely expensive even today, and they are used in small amounts only for highly specialized applications.
As early as 1960, Bacon et al., J. Appl. Phys. 34, 1960, 283-290 described the existence of carbon whiskers, which consist of a rolled-up graphene layer. The structure is referred to as scroll type. The production process described by Bacon is based on the evaporation of a carbon electrode in an arc (arc discharge).
Similar structures of carbon tubes, in which a cohesive graphene layer (so-called scroll type) or a broken graphene layer (so-called onion type) is the basis for the structure of the nanotube, were later also found by Zhou et al., Science, 263, 1994, 1744-1747 and by Lavin et al., Carbon 40, 2002, 1123-1130. These carbon nanotubes are present in a carbon black produced by the arc discharge process in admixture with many other carbon structures. Such scroll-type or onion-type carbon nanotubes cannot readily be separated or produced in pure form. Industrial production of such special forms does not therefore come into consideration.
Carbon nanotubes consisting of a single rolled-up graphene layer were later also produced by means of a pyrolysis process by Ruland et al., Carbon 41, 2003, 423-427. Dravid et al., Science 259, 1993, 1601-1604 and Feng et al., J. Phys. Chem. Solids, 58, 1997, 1887-1892 describe intermediate structures in which graphene layers are wound round a single thicker carbon nanotube of the so-called bucky type. Bucky type is a name for multi-wall carbon nanotubes with round closed ends of graphite, which have concentric closed graphite cylinders.
In all these processes for the production of scroll- or onion-type carbon tubes, the energy outlay is very high and the yield is low, which makes practicable or industrial production impossible.
The production of multi-wall carbon nanotubes in the form of seamless cylindrical nanotubes nested inside one another is today carried out commercially in relatively large amounts predominantly using catalytic processes. These processes conventionally exhibit a higher yield than the above-mentioned arc discharge process and other processes, and are today typically carried out on the kg scale (several hundred kilos/day worldwide). The MW carbon nanotubes so produced are generally somewhat less expensive than single-wall nanotubes and are therefore used in certain amounts as a performance-enhancing additive in other materials.
The object of the present invention is to develop a process for producing carbon nanotubes in even greater amounts, which exhibit at least the properties of the known MWCNT structures.
Furthermore, the CNT material is to have high purity in respect of metallic impurities and amorphous carbon, impurities which can lead to impairment of material properties on incorporation into matrix materials, for example into polymers. Furthermore, the product is to have in particular good pourability and especially is to be largely free of dust and is to have as high a bulk density of the CNTs as possible, in order to facilitate transport and handling both during production and during the filling of the CNT material into containers and its transfer into different containers, and during subsequent incorporation. A large inner surface area of the CNT material would also be particularly advantageous.
It was possible to achieve this object by the provision of a chosen catalytic gas-phase process by means of which, by the choice of special suitable catalysts and process conditions, novel carbon nanotube powders are formed which consist predominantly of carbon nanotubes consisting of one or more continuous graphite layers which are rolled up to form a tubular structure.